Hey Tintin, thanks a lot for bringing up this question. It's a pity that there is not much discussion about Hinduism even among Christians in India. I am a beginner in this research project, so I will share with you what little I know.
Ancient Hinduism did not even start off as a religion per se. It started off as man worshipping the 5 elements of nature - earth, water, fire, wind and sound. Slowly it came to be influenced through folklore and interactions with other religions to form Hinduism as we see it today. Today, Hinduism has 330 million gods and is followed by over a billion people. Rachel20 has explained much about this in detail in her posts.
One alternative explanation for the origin of Hinduism is given by those who claim that some gods in Hinduism originated from Christianity. According to them, when the gospel was brought to India the disciples of St. Thomas created small plays to teach the various Biblical truths. This is a social activity called 'street-play', which is still active in many villages today. The disciples created characters and used street-plays to carry the gospel to the illiterate masses. These street-plays and characters were passed down from generation to generation and formed part of the local folklore. Later, they also became some of the gods in present-day Hinduism. There are many proponents of this theory. In fact, there are some former temple priests who today worship Jesus but only worship Him through reciting the slokas ('sloka' - verse) in the Hindu scripts. I don't know if this is credible or not, but I thought I must share it with you.
My hypothesis is that the ancient languages of Tamil and Sanskrit have some connection with Judaism and subsequently, with Christianity. Whether the connection is significant or not, I don't know but here are some interesting points -
1. There are many Sanskrit words used in the Old Testament. For example, Numbers 21:8 speaks of mentions the word
"Nachash" for a serpent. The etymology of this word comes from Sanskrit, which is
"Nag", meaning snake/serpent.
2. This is part of my ongoing research, so it is only a hypothesis. But, some of the characters in the Bible have similar names (and probably genealogy/function) with certain characters in Hinduism too. Just as Judaism holds Abraham as the father of their faith, Hindus hold Brahma as the 'Origin, the One from whom everything began' (paraphrased). If you look at the etymology of the word Brahma, it comes from
"Brah" meaning, 'to grow or multiply in number'. Brahma is called the 'father of all men and exalted of all gods'. Similarly, there are other characters which can be compared between our Scriptures and Hinduism.
3. Tamil also has had some connection with Israel in the early days. For example, the verse I Kings 10:22 uses the word
"Thukkim" for peacock. This is the same word for peacock, as in the Tamil language. Furthermore, some scholars believe that the land of Ophir was in India.
In short, I believe that Hinduism is a religion devoted to the "Unknown God". They follow various practices and customs that are very similar to Judaism (animal sacrifices, smearing of ashes, etc.), but they don't know the intention behind each practice and custom. Perhaps God would raise up a scholar who can prove that Hinduism is indeed a more ritualistic version of Judaism. That would pave the way for many Hindus to come to Christ.